Parshat Bo, perek 12 pasuk 1 says:
“Hashem said to Moses and Aaron in the land of Egypt, saying,’ This month shall be for you in the beginning of the months’?”
Rashi comments on this verse saying that Nissan should be the first of the months while all the other months should be counted after the month of Nissan; Iyar is the second month, Sivan the third, and so on. Rashi parallels this pasuk with Shabbos. Shabbos is our sign and our everlasting covenant, and we use Shabbos to thank Hashem for the world around us, and to realize that Hashem has a hand in everything in this world.
We must believe that Hashem still continues to be the Creator and the Ruler of everything that happens to any human or animal. Knowing Hashem is the Creator and takes part in everything in this world, we must apply that to our everyday lives. Just like we would count the days of the week after Shabbos, we would also count the months of the year after the month of Nissan. The lesson of the plagues and Exodus as narrated in the Torah is an example of why Nissan is the first month. Similar to Shabbos, the month in which Exodus occurred was chosen to be the first of the months as a sign of Hashem’s ongoing rulership over all people and events.